We have previously shown that, in oxygenated rat kidney proximal convoluted tubules (PCT), activation of protein kinase C (PKC) by phorbol 12,13-dibutyrate (PDBu) directly stimulates Na(+)-K(+)-adenosinetriphosphatase (ATPase) activity. PKC modulation of Na(+)-K(+)-ATPase activity by phosphorylation of its alpha-subunit was the postulated mechanism. The present study was therefore designed to investigate the relationship between PKC-mediated phosphorylation of the catalytic alpha-subunit and the cation transport activity of the Na(+)-K(+)-ATPase. In a suspension of rat kidney cortical tubules, activation of PKC by 10(-7) M PDBu increased the level of phosphorylation of the Na(+)-K(+)-ATPase alpha-subunit and stimulated the ouabain-sensitive 86Rb uptake by 47 and 42%, respectively. Time and dose dependence of the PDBu-induced increase in Na(+)-K(+)-ATPase activity and phosphorylation was strongly linearly correlated. The effects of PDBu on phosphorylation and activity of Na(+)-K(+)-ATPase were prevented by GF-109203X, a specific PKC inhibitor, whereas H-89, a specific PKA inhibitor, was ineffective. These results demonstrate that PKC activation induces phosphorylation of the catalytic alpha-subunit of Na(+)-K(+)-ATPase, which may participate in the stimulation of its cation transport activity in the rat PCT.

Phosphorylation of the ␣-subunit of Na ϩ ,K ϩ -ATPase plays an important role in the regulation of this pump. Recent studies suggest that insulin, known to increase solute and fluid reabsorption in mammalian proximal convoluted tubule (PCT), is stimulating Na ϩ ,K ϩ -ATPase activity through the tyrosine phosphorylation process. This study was therefore undertaken to evaluate the role of tyrosine phosphorylation of the Na ϩ ,K ϩ -ATPase ␣-subunit in the action of insulin. In rat PCT, insulin and orthovanadate (a tyrosine phosphatase inhibitor) increased tyrosine phosphorylation level of the ␣-subunit more than twofold. Their effects were not additive, suggesting a common mechanism of action. Insulin-induced tyrosine phosphorylation was prevented by genistein, a tyrosine kinase inhibitor. The site of tyrosine phosphorylation was identified on Tyr-10 by controlled trypsinolysis in rat PCTs and by site-directed mutagenesis in opossum kidney cells transfected with rat ␣-subunit. The functional relevance of Tyr-10 phosphorylation was assessed by 1) the abolition of insulin-induced stimulation of the ouabain-sensitive 86 Rb uptake in opossum kidney cells expressing mutant rat ␣1-subunits wherein tyrosine was replaced by alanine or glutamine; and 2) the similarity of the time course and dose dependency of the insulin-induced increase in ouabain-sensitive 86 Rb uptake and tyrosine phosphorylation. These findings indicate that phosphorylation of the Na ϩ ,K ϩ -ATPase ␣-subunit at Tyr-10 likely participates in the physiological control of sodium reabsorption in PCT.

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The kidney medulla is exposed to very high interstitial osmolarity leading to the activation of mitogen-activated protein kinases (MAPK). However, the respective roles of increased intracellular osmolality and of cell shrinkage in MAPK activation are not known. Similarly, the participation of MAPK in the regulatory volume increase (RVI) following cell shrinkage remains to be investigated. In the rat medullary thick ascending limb of Henle (MTAL), extracellular hypertonicity produced by addition of NaCl or sucrose increased the phosphorylation level of extracellular signal-regulated kinase (ERK) and p38 kinase and to a lesser extent c-Jun NH 2 -terminal kinase with sucrose only. Both hypertonic solutions decreased the MTAL cellular volume in a doseand time-dependent manner. In contrast, hypertonic urea had no effect. The extent of MAPK activation was correlated with the extent of MTAL cellular volume decrease. Increasing intracellular osmolality without modifying cellular volume did not activate MAPK, whereas cell shrinkage without variation in osmolality activated both ERK and p38. In the presence of 600 mosmol/liter NaCl, the maximal cell shrinkage was observed after 10 min at 37°C and the MTAL cellular volume was reduced to 70% of its initial value. Then, RVI occurred and the cellular volume progressively recovered to reach about 90% of its initial value after 30 min. SB203580, a specific inhibitor of p38, almost completely inhibited the cellular volume recovery, whereas inhibition of ERK did not alter RVI. In conclusion, in rat MTAL: 1) cell shrinkage, but not intracellular hyperosmolality, triggers the activation of both ERK and p38 kinase in response to extracellular hypertonicity; and 2) RVI is dependent on p38 kinase activation.During diuresis and antidiuresis, the kidney medulla is exposed to large fluctuations of interstitial osmolarity (1), which challenge cell volume constancy. Cells of the medullary thick ascending limb of Henle (MTAL) 1 are of special interest, since they are the major contributor to the generation of the renal cortico-papillary osmotic gradient allowing urinary concentration in terrestrial animals. The first adaptive process occurring in response to extracellular hypertonicity-induced cell shrinkage is a regulatory cell volume increase (RVI). The RVI results from the stimulation of ion transporters which increase the intracellular ion content within minutes and partially restore the cellular volume from the initial cell shrinkage (2, 3). A second adaptive mechanism, in mammalian cells, is the induction of genes encoding proteins involved in the accumulation of intracellular "compatible osmolytes" within hours and days. These osmoprotective proteins are either enzymes, i.e. aldose reductase generating sorbitol from glucose, or organic osmolytes transporters, i.e. myo-inositol, taurine, glycerophosphocholine, and betaine (4). The intracellular signaling pathways mediating these adaptive mechanisms, especially the role of MAP kinases, are still incompletely understood. Mitogen-activ...

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